Refractory cores and methods of making the same

ABSTRACT

REFRACTORY CORES WHICH CAN BE USED IN INVESTMENT CASTING WITHOUR FIRST HAVING BEEN FIRED TO AN ELEVATED TEMPERATURE ARE MADE BY A PROCESS CHARACTERIZED BY THE STEPS OF: (1) MOLDING A CORE FROM REFRACTORY MIXTURE INCLUDING A SUBLIMABLE BINDER, (2) SUBLIMING THE BINDER FROM THE CORE TO PRODUCE A POROUS STRUCTURE, (3) IMPREGNATING THE PORE STRUCTURE WITH A HARDENING AND STRENGTHENING AGENT, AND (4) HARDENING THE MATERIAL USED FOR IMPREGNATION.

United States Patent O1 fice 3,686,006 Patented Aug. 22, 1972 3,686,006 REFRACTORY CORES AND METHODS OF MAKING THE SAME Robert A. Horton, Chesterland, Ohio, assignor to Precision Metalsmiths, Inc. No Drawing. Filed Dec. 2, 1970, Ser. No. 94,580 Int. Cl. B28b 7/28 US. Cl. 106-383 19 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION The present invention relates generally to refractory cores, and more specifically to methods of making refractory cores for use in metal casting processes, especially investment casting.

Preformed ceramic cores are used in investment casting, and to a lesser extent in other casting processes, to form holes, slots, etc. in the castings. The cores are used to make investment molds in several ways. Often the core is positioned within the pattern material injection die and molten wax or other pattern material is injected around the core to form the pattern. Disposable patterns of wax or other material are also made separately and the cores inserted into openings in the patterns. In either case the cores become incorporated into the investment molds which are formed around the patterns. The core is made to extend beyond the pattern at one or more locations and becomes embedded in the mold at these locations so that it is firmly held in position when the pattern is melted out of the mold. In other casting procedures, the cores are assembled directly into the molds, for example, molds of the cope and drag type, which are provided with suitable core prints in which the cores are seated.

The use of refractory cores as generally described above can enlarge the scope of metal casting processes to permit the manufacture of parts which otherwise would not be feasible and can often reduce casting costs by lowering rejection rates and simplifying molding operations. In spite of these advantages, the extensive use of refractory cores has been limited by several factors, including their low strength and hardness, their expense, and the special equipment and procedures required for manufacture.

The factors which heretofore have restricted the use of refractory cores are to a large extent associated with the conventional practice of firing the cores to an elevated temperature during their manufacture in order to sinter the refractory material and develop a ceramic bond. This practice of firing cores at high temperatures is undesirable for a number of reasons. It causes sintering shrinkage which makes it difficult to control dimensions and prevent warpage. It also reduces the porosity of the cores so that they are not readily permeable to the hot mold gases, and this results in gas entrapment in the castings. High temperature sintering of refractory cores can result in excessive strength which is undesirable during the casting operation. Excessive strength of the cores can cause hot tearing of the metal castings and can make it difiicult easily to remove the cores from the castings.

The conventional practice of firing cores to sinter the refractory requires special equipment, materials and operations which have contributed to the cost of core production. For example, high temperature furnaces are required for sintering the cores. In addition, it is customary to provide refractory saggers in which the cores are fired. Other conventional expedients include embedding the cores in granular refractory particles or providing individual refractory forms contoured to the shape of the cores in order to support them during firing.

SUMMARY OF THE INVENTION The present invention provides for the production of refractory cores by a process which eliminates the need for firing the cores prior to use. Cores made according to the new process can be incorporated directly into disposable patterns for use in investment casting without having first been fired to an elevated temperature as has been customary in order to provide the strength necessary to resist breakage and damage during storage, handling, shipping, machining, wax injection, and other operations.

In the preferred embodiment of the invention, the new core making process involves the steps of:

(1) preparing a mixture consisting essentially of comminuted refractory material as the major component, a sublimable binder and a non-sublimable binder,

(2) forming a core from the mixture,

(3) subliming the sublimable binder out of the core to produce a porous structure,

(4) impregnating the pore structure of the core created by removal of the sublimable binder with a hardening and strengthening agent, and

(5) hardening the material used for impregnation.

The sublimable binder used in the process is characterized by the capability of being changed from a solid to a flowable state, as by heating, and of being resolidified, and by a high vapor pressure of at least .1 mm. of Hg at the triple point under normal atmospheric pressure, whereby the binder can be readily sublimed from the core. Preferred sublimable binders are solids at room temperature and can be removed readily from the cores at normal room temperature. Such materials are characterized by a boiling point which is no higher than about 250 C., more preferably by a boiling point which is no higher than 200 C., and by a vapor pressure of at least .1 mm. of Hg at normal room temperature and atmospheric pressure. The non-sublimable binder has the characteristics of being compatible with the sublimable binder when it is in a fluid state, the capability of being hardened to impart strength to the cores formed from the mixture, and a sufficiently low vapor pressure that it will serve tohold the cores intact during sublimation of the sublimable binder and during impregnation. The preferred material used for impregnation is a refractory binder in a carrier liquid.

The step of hardening the impregnating agent is carried out in a number of ways in order to obtain cores which exhibit varying combinations of strength and hardness. In one procedure cores having hard surfaces and relatively soft interiors are produced by evaporating all or part of the carrier liquid. In another procedure cores having relatively uniform strength and hardness are produced by precipitating the refractory binder material in the pores of the core, as by freezing or gelling the impregnating agent. In still another procedure the impregnating agent is hardened by partially evaporating the carrier liquid and then precipitating the refractory binder material from the remaining carrier liquid in the pores of the core.

Cores made in accordance with the process of this invention exhibit the high strength and hardness which are necessary to resist breakage and damage during handling, shipping, wax injection operations, etc., and will produce excellent casting surfaces which are equal or superior to casting surfaces obtained with sintered cores. As described above, the properties of strength and hardness can be varied as desired to meet particular casting requirements. Another important advantage is that cores can be made with a high degree of dimensional accuracy. Since the impregnating agent is deposited in the pore structure formed by subliming one of the core binders, any shrinkage encountered is minimal compared to conventional procedures involving firing.

The elimination of the conventional firing step results in several additional advantages. High temperature furnaces, refractory saggers, specially made core-supporting forms, etc. which are typically used in sintering operations are not required. Foundries making their own cores do not require extra furnaces for core production and core suppliers do not require any furnaces at all. By eliminating sintering operations and the need for special equipment and materials used in carrying out such operations, the present invention makes it possible to substantially reduce the costs of core production.

Other advantages and a fuller understanding of the invention will be had from the following detailed description.

DESCRIPTION OFTHE PREFERRED EMBODIMENT The first step of the new process of making refractory cores involves preparing a moldable mixture consisting essentially of comminuted refractory material as the major component, a sublimable binder and a non-sublimable binder. Any of the refractory materials which are conventionally used for making ceramic cores can be employed in preparing the mixture. Typical refractories include fused and crystalline silica, zircon, zirconia, alumina, calcium zirconate, various aluminum silicates, tricalcium phosphate, and the like.

A sublimable binder is included in the mixture so that a porous structure can be produced by subliming the binder from a core molded from the mixture. The term sublimable binder is used to mean any material which has a sufliciently high vapor pressure that it can be sublimed at a temperature below its softening point and below the softening point of the non-sublimable binder. The materials which are useful as the sublimable binder in the process of the present invention are generally characterized by the capability of being changed from a solid to a flowable state and of being resolidified, and by a vapor pressure of at least .1 mm. of Hg at the triple point under normal atmospheric pressure. It has been found that materials having a vapor pressure of at least .1 mm. of Hg at the triple point under normal atmospheric pressure can be sublimed from the molded cores in a reasonable length of time either with only moderate heating or with no heating at all. The more preferred materials have a triple point above normal room temperature and can be sublimed at room temperature in order to avoid the danger of distorting the cores by excessive heating. Such materials have a vapor pressure of at least .1 mm. of Hg at room temperature under atmospheric pressure and have a boiling point which is no higher than about 250 C. and which more preferably is 200 C. or less. The preferred sublimable binders are solids at room temperature in order to avoid the necessity of refrigerating to cause solidification, and are further characterized by the capability of being melted to a flowable state upon heating and of resolidifying upon cooling.

Exemplary materials which exhibit the foregoing characteristics and are capable of serving as sublimable binders include aliphatic and substituted and unsubstituted aromatic compounds. Paradichlorobenzene is a particularly suitable compound which has a conveniently low melting point, good molding properties, low cost, and the capability of being readily sublimed from molded cores. Other exemplary materials which have been found satisfactory include urethane (ethyl carbamate), acetamide, naphthalene, benzoic acid, phthalic anhydride, camphor, anthracene, and parachlorobenzaldehyde. Other exemplary materials which display similar properties and are contemplated for use include l-bromo-4-chlorobenzene, paradiamethoxybenzene, paradibutoxybenzene, and crotonic acid. The vapor pressures of materials such as paradichlorobenzene, urethane, acetamide, camphor, and naphthalene are sufficiently high that these materials can be sublimed at room temperature under normal atmospheric pressure or at pressures below atmospheric. Materials such as phthalic anhydride, benzoic acid, and anthracene have a lower vapor pressure at room temperature and, while useful for the purposes of the present invention, require heating to facilitate sublimation.

The primary function of the non-sublimable binder is to hold the cores intact when the sublimable binder has been removed and before the cores have been impregnated with the strengthening and hardening agent. The term non-sublimable binder is used to mean any material which is capable of being hardened, preferably at room temperature, to impart the necessary strength to cores molded from the mixture and which has a sufficiently low vapor pressure that it will not melt, sublime or evaporate at the temperatures and under the pressures employed to effect removal of the sublimable binder. The useful nonsublimable binders are further characterized by being miscible with the sublimable binder when it is in a fluid state. Exemplary materials exhibiting these characteristics are organic binders including resinous and plastic materials. Examples of suitable resins are polymerized rosins, hydrogenated rosin esters, petroleum hydrocarbon resins, coumarone-indene resins, ester gums, polyterpine resins, low molecular weight styrene polymer resins, chlorine polyphenyl resins, gilsonite, abietic acid, shellac, and silicone resins- Examples of suitable plastic materials are polystyrene, ethyl cellulose, polyvinyl acetate and polyvinyl alcohol.

In addition to performing its primary function of imparting sufficient strength to the molded cores to hold them intact after removal of the sublimable binder and before impregnation, the non-subliming binder or combination of such binders may be selected to impart other desired characteristics, such as improving fluidity in the molding process, preventing separation of the sublimable binder under pressure, reducing the set-up time of the core mixture, and improving the molded strength of the cores. The silicone resins are particularly advantageous, since they serve to increase the hot strength of the molded cores. Ethyl cellulose also is especially useful, since it acts to prevent bleeding or separation of the sublimable binder under pressure, as during injection molding.

The total binder content (subliming and nonsubliming binders) may vary within a range of about 10 to 40% by weight of the mixture, and more preferably from about 20 to 30% by weight of the mixture. The subliming binder should be at least one-half of the total binder content, and usually is at least of the total binder content. The amount of the subliming binder used in preparation of the mixture may vary widely over a range of from about 10 to 35% by weight of the mixture with a more preferred range being from about 2 to 5.5% by weight. The amount of the non-subliming binder may vary from about 0.8% to 11% by Weight of the mixture with a more preferred range being from about 2 to 5 5% by weight.

A subliming binder content near its lower limit is generally used in conjunction with the more dense refractories, and in such mixtures the non-subliming binder is usually required to be at or above the mid-point of its range in order to provide sufficient fluidity for melding of the cores. A subliming binder content near its upper limit is generally used in conjunction with the lighter refractories or where very fluid mixtures are desired for particular core-forming operations. Generally, the nonsub'liming binder will be near the lower limit of its range when the subliming binder content is near the upper limit of its range. The minimum amount of the non-subliming binder is that required to impart sufiicient strength to the cores that they can be handled successfully in the impregnating operation. Higher amounts may be desirable to provide extra strength to the impregnated cores or to modify the injection or plastic flow propertles of the mixture so as to facilitate the core-forming operation. The subliming binders typically have sharp melting points and melt to thin liquids. As a result the forming properties of the mixture based solely on the subliming binders are often poor. The forming properties of the mixture are greatly improved by a proper selection of the non-subliming binders.

The mixing of the comminuted refractory and binders can be accomplished by any suitable procedure. For example, when using the preferred materials, the binders may be heated and blended together in a suitable mixer and the comminuted refractory material added to the blend until the mixture is of the desired consistency for the subsequent molding operation. If desired, the refractory material may be warmed prior to adding it to the blended binders. The resulting mixture may be molded while hot or it may be solidified and subsequently melted for the core-forming operation.

The second step of the process is that of forming or molding cores from the core mixture. Any of the wellknown core forming techniques can be employed, such as injection molding, casting or pouring, transfer molding, extruding, etc. 'In one preferred example of the invention, the cores are quickly and inexpensively formed by injection molding using conventional plastic irnectron molding machines. For purposes of injection molding, the core mixture prepared as described above is cooled and granulated, as by stirring the melt as it cools to solidification, and the granules are charged into the in ection molding machine in the conventional manner.

In the third step of the process, the sublimable binder is removed from the molded cores. An outstanding advantage of the sublimable binders used in the process of this invention, namely, materials having a vapor pressure of at least .1 mm. of Hg at the triple point, and more especially materials which have a vapor pressure of .1 mm. of Hg at room temperature and a boiling point which is no higher than 250 C. and preferably is 200 C. or less, is that the binders can be sublimed from the molded cores with very little shrinkage occurring even when the subliming binder is present in large amounts. Many of the useful sublimate binder-s will tend to escape from the molded cores upon standing at room conditions. In the case of very thin cores, all of the sublimable binder may be lost from the mold core in this manner within a reasonable length of time. Preferably, however, sublimation is facilitated by subjecting the cores to a partial vacuum, as by placing the cores in a chamber connected to a vacuum pump, or by moderately heating the cores, or by a combination of moderate heating and applying a partial vacuum. The procedure of subjecting the cores to reduced pressure in a vacuum chamber is extremely effective and avoids the possibility of warpage. While heating of the cores is useful to facilitate sublimation, it must be done carefully, since the cores can readily distort under excessive heat.

After the subliming binder has been removed, the cores are ready for the impregnating operation which is the fourth step of the new process. This step involves at least partially impregnating the pores of the core formed by removal of the sublimable binder with a strengthening and hardening agent. The material used to impregnate the pore structure of the cores is selected to serve three functions. It increases the strength and hardness of the sublimed core so that it can readily withstand the stresses encountered in handling, shipping, patten injection, and other precasting operation. Secondly, it provides a continuous bond to hold the core together when an investment mold containing the core is fired preparatory to casting metal into the mold. At normal mold firing temperatures ranging from about 1600 F. to about 200 0" F., the non-sublimable binder in the core may be eliminated so that the core strength depends entirely upon the impregnating material. A third and related function of the material which is impregnated into the pore structure is to provide sufficient strength to enable the core to resist the stresses encountered when molten metal is cast into the mold against the core.

The materials used for impregnation in accordance with this invention are any of the refractory binders commonly employed in investment casting processes. Such binders are distinguished by the capability of being dried, chemically gelled or precipitated to form a continuous, preferably noncrystalline, bonding structure. The refractory binders may be classified as being either colloidal type or solution-type binders. The class of colloidal-type binders is broad and includes hydrolyzed alkyl silicates, silica sols or dispersions, and dispersions or suspensions of various inorganic silicates. Examples of hydrolyzed alkyl silicates include isopropyl silicate, the various ethyl silicates, and other lower alkyl silicates. Examples of in organic silicates include lithium polysilicate, sodium silicates, potassium silicates, quaternary ammonium silicates, and the like. The class of solution-type binders is further characterized in that the binders are metal salts. Examples of this class include solutions of sodium silicates, mono-aluminum phosphate, potassium silicates, aluminum acid phosphate, zirconium acetate, ammonium zirconyl carbonate, aluminum formate, and the like. Of this group, mono-aluminum phosphate is considered to be particularly satisfactory.

The concentration of the binders in the medium in which the binders are dissolved or dispersed, i.e., water or alcohol, may vary over a wide range. By way of example, binder concentrations may range from 5 to 60% by weight or higher. In general, a lower range of from about 5 to 35% by weight applies to solution-type binders and a higher concentration range of from about 15 to 60% by weight or higher applies to the colloidal-type binders. The upper concentration of the solution-type binders is frequently set by the viscosity of the solution. Too 'viscous a liquid will not penetrate the pore structure of the cores. The upper limit for the concentration of the colloidaltype binders may be set by the need to obtain a core which will be sufficiently permeable to the mold gases which are produced during casting. When the cores made in accordance with this invention are dried and investment molds containing the cores are heated prior to casting, the liquid medium in which the impregnating agent is dispersed or dissolved is removed from the cores to obtain the desired core permeability. Since the binders themselves are usually at least twice as dense as the medium in which they are dispersed or dissolved, the concentration of the binder is lower on a volume basis than on a weight basis. Hence, even at high concentrations, such as a 60% by weight binder concentration, satisfactory permeability can be produced in the final core following removal of the liquid medium. When employing the colloidal-type binders, the individual particles should be smaller than the pores of the cores which are to be impregnated. Otherwise the impregnating agent or binder will be filtered out on the surface of the core. Satisfactory results have been obtained using colloidal-type binders which have a particle size less than 70 millimicrons.

Colloidal silica sols are especially preferred for use as the impregnating agent because of the convenient procedures which can be employed to precipitate the silica. The preferred colloidal dispersions may be stabilized both by acid and alkali. Acid-stabilized sols can be gelled by the addition of alkali materials to raise the pH above about 5. Alkali-stabilized sols become unstable in the presence of foreign ions and the gel time can be conveniently regulated by adding a proper amount of any of a wide variety of salts, including soduim chloride, sodium acetate, ammonium acetate, ammonium chloride, and the like. Aqueous colloidal silica dispersions can be frozen to precipitate the silica which remains in the precipitated state when the material is subsequently thawed.

The time and manner of impregnating the cores can be varied widely. In many instances complete impregnation of the core structure is desired, and in such cases the minimum impregnating time is that required to achieve the desired penetration of the pores. The operation can be readily carried out by immersing the cores in the liquid impregnating agent under a partial vacuum in order to assure complete penetration of the pore structure in the shortest length of time. If desired, a suitable wetting agent may be added to the liquid impregnating agent in order to promote penetration of the pores of the cores. The use of a vacuum is not essential and satisfactory results can be obtained in most instances by dipping the cores under normal atmospheric pressure. In some cases less than complete penetration of the core structure my be satisfactory and even desirable. For example, when the non-subliming binder can be softened or otherwise attacked by the impregnating agent, the time of impregnation is accordingly limited. Complete impregnation of the core structure also may not be necessary in situations where increased surface hardness rather than strength is of primary importance.

In the fifth step of the process, the liquid impregnating agent is hardened. The hardening step can be carried out by different procedures in order to obtain varying combinations of core strength and hardness, as may be desired because of particular core shapes and/ or to meet particular casting requirements. According to one such procedure hardening of the liquid impregnating agent is accomplished by evaporating the carrier liquid, as by air drying.

When the liquid impregnating agent is hardened by evaporation of the liquid medium in which the binder, used for impregnations is dispersed or dissolved, the binder material tends to migrate toward the core surfaces with the evaporating liquid and is deposited in a layer at the surfaces. This results in an extremely hard surface on the core and leaves the interior relatively soft and weak. This may be desirable in some instances, as when the cores are to be subjected to extremely errosive conditions.

In other cases a more uniform depositing of the binder material used for impregnation throughout the pore structure is desired. For example, an accumulation of the binder material in the outer portion of cores having large flat surfaces may not be desired because of the tendency of such cores to spall when molds containing the cores are heated preparatory to casting. In such cases, a preferred hardening procedure is to gel or precipitate the binder material used for impregnation within the pores before the liquid medium has had a chance to migrate to the core surface and evaporate. Such a procedure results incores which are stronger than cores produced by air drying of the impregnating agent, but which may be less hard on the surface. The two hardening techniques also can be combined to vary the core properties by first permitting partial evaporation of the carrier liquid followed by gelling or precipitation of the remaining impregnating agent in the pores.

The impregnating agent can be caused to deposit uniformly throughout the pore structure of the cores in a number of ways depending upon the particular material used for impregnation. As mentioned above, the preferred aqueous colloidal silica dispersions can be frozen to precipitate the silica which remains in the precipitated state when the material is subsequently thawed. Freezing of cores impregnated with aqueous colloidal silica has been found to be a particularly convenient and satisfactory technique for uniformly despositing the silica throughout the core.

Another expedient is to add gelling agents to the liquid impregnating agent to render it unstable so that it will gel and harden spontaneously in a controlled length of time. The time for gelling and hardening can be adjusted to permit the impregnating operation to be carried out successfully. Another expedient is to dip the impregnated cores into a liquid gelling agent which is capable of gelling the impregnating material on contact. Still another technique is to include a gelling agent in the core itself, the potency of the gelling agent being adjusted so as to permit impregnation to be carried out successfully followed by gelling of the impregnating agent within the pores in a controlled length of time.

After the impregnating agent has been hardened in place by any of the above techniques or by any other procedure, the cores are preferably dried to remove free water or other carrier liquid. The drying step can be omitted in some instances, but is generally found to be advantageous in order to obtain maximum strength. Vacuum drying or air drying with moderate air circulation are satisfactory procedures. Controlled oven drying also can be used.

The process of this invention is illustrated by the following examples.

EXAMPLE 1 A core batch was prepared of the following composition, percentages being specified as percentages by weight:

Percent Refractory blend 74.6 Paradichlorobenzene 21.0 Shellac 3.4 Ethyl cellulose (10 cps.) 1.0

The refractory blend consisted of the following ingredients in amounts by weight:

Percent 325 mesh zircon 58.8 Fused silica, 99% minus 200 mesh U.S. sieve, 75%

minus 325 mesh 39.2

Nepheline syenite 2.0

The core batch was prepared by first blending together the refractory powders and then warming the refractory blend to approximately 200 F. The paradichlorobenzene, shellac and ethyl cellulose were melted and blended separately and all ingredients were combined using a mixture having a whip type agitator. The core batch was granulated by cooling to solidification while continuing to stir.

A variety of cores for commercial parts were molded from the core batch using a standard plastic injection molding machine. The machine was of the plunger-type with horizontal injection and vertical mold opening and closing. Injection of the core material was along the parting line. The injection conditions were as follows:

Cylinder temperature: 180 F.

Nozzle temperature: 180 F.

Injection time: 6 seconds Cycle time: 12-60 seconds depending on core size Injection pressures: 1200-1700 p.s.i. depending on core configuration The step of removing the paradichlorobenzene was accomplished by placing the injection molded cores in a vacuum chamber under a reduced pressure of approximately 27 inches of mercury below atmospheric pressure for a length of time suitable to remove substantially all of the subliming binder. The time for removing the subliming binder varied from four to ten hours depending upon the sizes and shapes of the cores.

After the subliming operation had been completed, the cores were impregnated under a vacuum with an aqueous colloidal silica sol (SYTON FM, manufactured by Monsanto Chemical Co.). The sol was a fine particle size sol containing 30% by weight colloidal silica particles having an approximate particle diameter of 7-8 millimicrons. The time for impregnation varied from 45 to 60 minutes. Subsequent tests have shown that much shorter impregnation times are practical and often preferable. As little as two minutes is often suflicient and even a shorter time may be feasible for very small core Following impregnation, the cores were rinsed quickly in water and placed in the freezing compartment of a standard home refrigerator until frozen hard. The cores were then removed and allowed to thaw and dry in a gentle current of air. The cores were ready for use immediately after drying, but also could be stored for indefinite periods before use, if desired.

The cores were incorporated into disposable patterns in two ways. Simple cores were inserted directly into openings in the patterns. In instances where this was not possible or desired, the cores were inserted into the pattern injection mold and wax was injected around the cores. The patterns containing the cores were assembled into set-ups of the type conventionally used in making ceramic shell molds, and the set-ups were processed according to normal ceramic shelf practice.

The ceramic shell molds were fired to temperatures ranging from 1600 F. to 2000 F., and various molten steel alloys were cast into the molds against the cores to produce castings. The steel alloys included types SAE -l, 6150, 8620, 302 stainless and 304 stainless, and were poured at temperatures of from 2900" F. to 3000 F. The cores were removed from the castings in a molten caustic bath.

The cores functioned in an entirely satisfactory manner and produced casting surfaces which were equivalent to or better than the casting surfaces produced against the ceramic shell molds.

EXAMPLE 2 A core batch was prepared of the following composition, percentages being specified as percentages by weight:

Percent Refractory powder 74 Paradichlorobenzene 21 Hydrogenated rosin ester (Staybelite Resin) 5 The refractory blend consisted of the following ingredi- The paradichlorobenzene and the hydrogenated rosin ester were melted and blended together and then the refractory powder was added to the blend. The mix was heated and stirred until it was of a thin, creamy consistency. It was then poured with gentle vibration into an aluminum form having the shape of the desired core. After solidification, the core was removed from the aluminum form and was placed in a vacuum chamber overnight at room temperature under a reduced pressure of 29 inches of mercury below atmospheric pressure in order to remove the paradichlorobenzene by sublimation. All of the subliming binder had been removed by the following day, as determined by weight loss measurements.

The core was impregnated with an acid-stabilized aqueous colloidal silica sol. The sol contained approximately 34% by weight silica having an average particle size of about 16-22 millimicrons. The impregnating operation was carried out under a vacuum of 25 inches of mercury below atmospheric pressure within a period of five minutes. Following impregnation, the core was given a quick water rinse and was placed in a freezing compartment of a refrigerator and frozen for a period of five hours. The core was then removed from the freezer and placed in front of a fan at room temperature to thaw and dry.

10 EXAMPLE 3 A core batch was prepared and cores were injection molded in the manner and under the conditions described in Example 1. The batch was of the following composition, percentages being specified as percentages by weight:

Percent Paradichlorobenzene 21 Shellac .4 Ethyl cellulose (20 cps.) 1 Refractory blend 74.6

. The refractory blend consisted of the following ingredients in amounts by weight:

Percent Zircon powder, 325 mesh 58.2 Fused silica powder, 99% minus 200 mesh U.S.

sieve, 75% minus 325 mesh 38.8

Nepheline syenite 3 EXAMPLE 4 A core batch was prepared of the following composition, percentages being specified as percentages by weight:

Percent Ethyl carbamate 17 Coumarone-indene resin 5 Refractory blend 78 The refractory blend was 60% by weight 325 mesh zircon, and 40% by weight fused silica as specified in previous examples.

The binders were melted and blended together and the refractory blend was stirred into the melt. The mixture was then vibrated into an aluminum form. After solidification the core was removed from the aluminum form, cooled, and subjected to a vacuum to remove the ethyl carbamate.

The core was then impregnated under a vacuum with an aqueous colloidal silica sol as described in Example 1. Following impregnation, the core was frozen in a refrigerator and then thawed and air dried in front of a fan at room temperature.

.EXAMPLE 5 Nepheline syenite 2.5

The mixture was cooled and granulated. The granules were charged into a standard plastic injection molding machine as described in Example 1 operating at a nozzle and cylinder temperature of F. and a pressure of approximately 1200 p.s.i. to produce injection molded cores.

The paradichlorobenzene was removed from the cores under a vacuum at room temperature, and the cores were impregnated under a vacuum within a period of five minutes using the aqueous colloidal silica sol described in Example 1.

1 1 EXAMPLE 6 A core batch was prepared of the following composition, percentages being specified as percentages by weight:

Percent Anthracene 21 Shellac 3.4 Ethyl cellulose (20 cps.) 1 Refractory blend 74.6

The refractory blend consisted of:

Percent by weight 325 mesh zircon 58.8 Fused silica, 99% minus 200 mesh U.S. sieve, 75%

325 mesh 39.2 Nepheline syenite 2 The mixture was heated to a pouring consistency and poured into a steel mold. After the core had solidified, it was placed in a forced air oven and heated slowly from room temperature to a temperature of 300 F. for approximately 40 hours by which time all of the anthracene had been sublimed from the core. The core was then impregnated with an aqueous colloidal silica sol under a vacuum for five minutes as described in Example 1.

It will be apparent from the foregoing that the invention has provided a new process for making refractory cores characterized in that the cores are not required to be fired prior to use. It will also be apparent that the process can be quickly and expeditiously carried out and that it results in several economic advantages, including the elimination of the need for special high temperature furnaces and associated equipment heretofore used in the production of ceramic cores by firing.

Many modifications of the invention will be apparent to those skilled in the art in the light of the foregoing disclosure. Therefore, it is to be understood that, within the scope of the appended claims, the invention can be practiced otherwise than as specifically described.

What is claimed is:

1. In a method of making refractory cores by a pro cedure including:

preparing a mixture comprising comminuted refractory material as the major component,

a first binder characterized by the capability of being changed from a solid to a flowable state and of being resolidified, and by a vapor pressure of at least .1 mm. of Hg at the triple point under normal atmospheric pressure of substantially one atmosphere, and

a second binder having the characteristics of being miscible with said first binder when it is in a flowable state, the capability of being hardened to impart strength to cores formed from said mixture, and a vapor pressure lower than that of said first binder so that said second binder will hold molded cores intact during sublimation of said first binder,

forming an unfired core from said mixture,

subliming said first binder from the unfired core to produce a porous structure,

the improvement wherein the core is strengthened and hardened without being fired to sinter the refractory material, said improvement comprising the steps of:

(a) at least partially impregnating the pore structure of the unsintered core with an impregnating agent consisting essentially of a refractory binder in a carrier liquid, and

(b) hardening said impregnating agent to impart strength and hardness to the unsintered core.

2. The improvement as claimed in claim 1 wherein said impregnating agent is a colloidal dispersion of inorganic material.

3. The improvement as claimed in claim 1 wherein said impregnating agent is a colloidal dispersion of silica.

4. The improvement as claimed in claim 1 wherein said impregnating agent is a colloidal dispersion of silica.

5. The improvement as claimed in claim 1 wherein said impregnating agent is a solution of one or more metal salts capable of drying to a continuous bonding structure.

6. The improvement as claimed in claim 1 wherein said step of hardening said impregnating agent is carried out by a procedure which includes at least partially evaporating the carrier liquid and allowing at least part of the refractory binder material to migrate toward the surface of the core.

7. The improvement as claimed in claim 1 wherein said step of hardening said impregnating agent is carried out by a procedure which includes at least partially evaporating the carrier liquid while allowing part of the refractory binder material to migrate toward the surface of the core, and then precipitating the refractory binder material from the remaining carrier liquid in the pores of the core.

8. The improvement as claimed in claim 7 wherein said refractory binder material is caused to deposit in the pores of the core during said step of hardening said impregnating agent.

9. The improvement as claimed in claim 1 wherein said step of hardening said impregnating agent is carried out by a procedure which includes freezing said impregnating agent in the pores of the core to precipitate said refractory binder material.

10. The improvement as claimed in claim 1 wherein said step of hardening said impregnating agent is carried out by a procedure which includes gelling said impregnating agent in the pores of the core.

11. The improvement as claimed in claim 10 wherein the impregnated core is dipped into a liquid gelling agent.

12. The improvement as claimed in claim 10 in which the material used for impregnating the core includes a gelling agent.

13. The improvement as claimed in claim 10 in which said impregnating agent is gelled by a gelling agent included in the mixture from which the core is formed.

14. In a method of making refractory cores by a procedure including:

preparing a mixture comprising:

comminuted refractory material as the major component,

a first organic compound having the characteristics of being a solid at room temperature, the capability of being melted to a fiowable state upon heating and of resolidifying upon cooling, and a vapor pressure of at least .1 mm. of Hg at the triple point under normal atmospheric pressure of substantially one atmosphere, and

a second organic compound having the characteristics of being a solid at room temperature, a melting point higher than that of said first compound, the capability of being melted to a flowable state upon heating and of being hardened upon cooling to impart strength to cores molded from said mixture, the capability of being miscible with said first compound when heated, and a vapor pressure lower than that of said first compound such that said second compound will hold molded cores intact during sublimation of said first compound,

forming a core from said mixture, and

subliming said first compound out of the core at a temperature below the softening points of said compounds to produce a porous structure,

the improvement wherein the core is strengthened and hardened without being fired to sinter the refractory material, said improvement comprising the steps of:

(a) at least partially impregnating the pores of the unsintered core with an impregnating agent which consists essentially of a refractory binder in a carrier liquid and which is capable of being hardened to form a continuous bonding structure, and

13 (b) hardening the impregnating agent to deposit the refractory binder material in the pore structure of the unsintered core to impart strength and hardness thereto.

15. The improvement as claimed in claim 14. wherein said step of impregnating said core is carried out in a partial vacuum.

16. The improvement as claimed in claim 14 wherein said impregnating agent is selected from the class consisting of colloidal dispersions of inorganic material and solutions of one or more metal salts.

17. The improvement as claimed in claim 14 wherein said step of hardening said impregnating agent is carried out by a procedure which includes drying the core to evaporate at least part of the carrier liquid.

18. The improvement as claimed in claim 14 wherein said impregnating agent is an aqueous colloidal silica sol, and wherein said step of hardening said impregnating l4 agent is carried out by a procedure which includes freezing the sol in the pores of the core.

19. The improvement as claimed in claim 14 wherein said step of hardening said impregnating agent is carried out by a procedure which includes gelling at least part of the impregnating agent in the pores of the core.

References Cited UNITED STATES PATENTS 3,236,665 2/1966 King 106-69 3,330,892 7/1967 Herrmann 264-63 3,346,680 10/1967 Bush 264-44 LORENZO B. HAYES, Primary Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECHN Patent NO. 3 686 006 Dated Aucrust 22 1972 Inventor(s) Robert A. Horton It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 63, delete "2 to 5.5%" substitute -l8 to 25%- Column 5, line-74, delete 'patten" substitute -pattern- Column 5, line 75, delete "operation." substitute operations.-

Column 7, line 23, delete "my" substitute -may- Column 12, lines 1 and 2, delete (claim 4) in its entirety substitute:

4. The improvement as claimed in claim 1 wherein said impregnating agent is an aqueous colloidal silica sol.

Signed and sealed this 9th day of January 1973..

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents- FORM PO-IOSO (10-6 USCOMM-DC 60376-P69 I US. GOVERNMENT PRINTING OFFICE I969 O-366334 

